On the other hand, the selection of a suitable vacuum state is also rather involved process since there are many inequivalent possibilities to choose. The only requirement we need to fulfill is the Hadamard condition [1]} or equivalently for FLRW spacetimes, the adiabatic condition [2]}. This condition determines the ultraviolet or short distance behavior of the vacuum state and ensures that the full renormalized stress-energy tensor and the two point function for this vacuum state is finite.
